chore(feed): Phase 2 #2 backend cleanup — dead return, dup helpers, shared const

- feed.py _filtered_query: drop the dead 2nd return element (status_expr was
  used only internally for WHERE filters; all 3 callers discarded it as _status).
  Now returns (query, rank_expr); fixed the stale docstring.
- youtube/client.py: extract _iter_playlist_items() — iter_my_playlist_video_ids
  and iter_playlist_items_with_ids were near-identical playlistItems paging loops
  (jscpd [173-187]≈[267-281]); both now map over the shared generator.
- sync/videos.py: extract _apply_video_batch() with an on_missing callback —
  enrich_pending and refresh_live shared the fetch-map-apply skeleton, differing
  only in the query and how they retire rows YouTube omits.
- models.py: add LIVE_OR_UPCOMING = ("live","upcoming"); replace the 4 duplicated
  copies (feed.HIDDEN_LIVE, search._LIVE_HIDDEN, videos.py + channels.py inline).

Behavior-neutral. ruff clean on touched files, localdev boots, feed/worker healthy.

backend/app/routes/search.py

@@ -24,6 +24,7 @@ from app import quota, sysconfig
 from app.auth import require_human
 from app.db import get_db
 from app.models import (
+    LIVE_OR_UPCOMING,
     BlockedChannel,
     Channel,
     Playlist,
@@ -45,8 +46,6 @@ router = APIRouter(prefix="/api/search", tags=["search"])
 
 # search.list costs this many quota units per page.
 SEARCH_COST = 100
-# live_status values we never surface from a live search (was_live VODs are real content).
-_LIVE_HIDDEN = ("live", "upcoming")
 
 
 def _classify_shorts(db: Session, videos: list[Video]) -> None:
@@ -165,7 +164,7 @@ def _ingest_candidates(
     # 4) Confirm/deny Shorts (no quota) so none enter via search.
     _classify_shorts(db, videos)
 
-    keep = {v.id for v in videos if not v.is_short and v.live_status not in _LIVE_HIDDEN}
+    keep = {v.id for v in videos if not v.is_short and v.live_status not in LIVE_OR_UPCOMING}
     return [vid for vid in ids if vid in keep]